Hello & welcome, funlog !
I have 1.00h, which is not supported. Can I load either f or g onto my camera safely so it will work, without screwing things up? I looked at the cannon site and cannot find any firmware for that model.
No, the CHDK versions for the SD550 1.00f or 1.00g will not work on your 1.00h version.
You can give it a try, but it was reported in the CHDK wikia that it will not work; when the camera crashes, just remove the batteries...